Throwing the slider harder means that Buchholz is basically using the pitch as a cut fastball, which he can then throw with more spin, resulting in a slower pitch with more break. If you look at the data, the pitch is most effective in the 86 mph range, causing the most whiffs and resulting in the most runs prevented. However, it’s not as simple as only relying on that version of the pitch. One reason it’s so effective is that hitters may be looking for the hard slider, and their timing can be upset when Buch pulls the string a little bit.

Apostoleris is right in that the slider really became a go-to pitch against lefties in 2010, especially inside. Look at the difference between 2009 and 2010 here. The cutter looks inside to a southpaw, then it bends in over the inner part of the plate.

Buchholz credits Jon Lester with helping him develop this pitch. Over the past few years, former pitching coach John Farrell introduced just about every Sox pitcher to the cut fastball as a way of easily adding a different looking pitch which could still be thrown for strikes, so this change is not that surprising.